1. Field of the Invention
This invention relates to a method of refining wood pulp; more especially the invention relates to such a method in which pulp consistency in the refiner is adjusted by controlled addition of dilution water to the refiner.
In a preferred embodiment, the present invention relates to a method for controlling TMP (thermomechanical pulp) refiners by adjustment of the refining intensity. Pulp consistencies in the refiner are controlled and adjusted to achieve stable refining intensity and to compensate for disturbances such as the ones associated with changes in production rate.
2. Description of the Prior Art
The quality of the pulp in thermomechanical pulp (TMP) refining is very much a function of the applied specific energy defined as the energy per tonne of production. The conventional approach to control pulp quality is therefore to adjust the specific energy either through changes in refiner motor load or through changes in refiner throughput, Owen J. et al “A practical approach to operator acceptance of advanced control with dual functionality. Proceedings Control Systems 98, Porvoo, Finland”.
Pulp quality also depends on the rate at which this energy is applied as expressed by the refining intensity or the specific energy per bar impact, Miles K. “A Simplified Method for calculating the residence time and refining Intensity in a chip refiner” Paperi ja Puu, 73(9):852–857 (1991)”. In practice, at a given specific energy, this refining intensity varies with pulp consistency. Pulp consistency affects the pulp residence time which itself is inversely proportional to the refining intensity. In an increasing number of installations the consistency of the pulp, as measured or estimated in the blow line, is controlled by adjusting the flow rate of dilution water into the refiner. Such consistency control helps to maintain discharge consistency in the appropriate range for the good operation of the refiner.
In large modern TMP refiners such as the Sunds CD 82 or some of the CD 76 refiners operating at very high refining consistency, there are up to three possible dilution flows that can be adjusted to change pulp consistency (as shown in FIG. 1): the infeed dilution or water added to the pulp or the chips before the refining zones, dilution water added to the flat zone of the refiner, and in some modern installations, the dilution water added to the conical zone. The purpose of adding dilution water in the conical zone is to reduce the occurrence of very high consistencies at the periphery of the plates and the associated plugging of the plates.
Although pulp consistency varies and normally increases from the refiner inlet to the refiner discharge or blow line, the term refiner pulp consistency conventionally denotes the consistency of the pulp at the refiner discharge. This pulp consistency is either measured on manual samples, estimated using predictive models, or measured on-line using commercially available sensors. In an increasing number of installations the consistency of the pulp is controlled through a single control loop where the three mentioned flow dilutions (in-feed, flat zone and conical zone dilution) are manipulated according to an established ratio (as illustrated in FIG. 2). The single loop consistency control scheme of the prior art has many limitations; one of them is its effect on specific energy. Indeed small changes in in-feed dilution or in flat zone dilution required for consistency control have significant impact on refiner motor load and much more so than changes in conical zone dilution. Another limitation of the single loop consistency control scheme is that the same discharge consistency can be obtained with different distributions of dilution water flows among in-feed, flat zone and conical zone dilutions. On the other hand, refining intensity and pulp quality will be different at these different distributions, a source of problems if not properly recognized. This explains why a refining condition that is evaluated only in terms of specific energy and blow line consistency can produce very different pulp properties.
This problem is partly addressed in U.S. Pat. No. 6,778,936 B2 where consistency profile is estimated using temperature sensors and a refining zone consistency is controlled either by manipulation of a dilution flow or by changing the refiner feed rate. However, in this previous U.S. patent no distinction has been made in the use of dilution water added before or during refining for consistency control. Only one consistency is being controlled. The objective there was to stabilize refining consistency not to adjust the target consistency for quality control. For example, there is no mention of the need to adjust refining consistency as a function of production rate to overcome loss of certain pulp properties. The same issue of quality loss due to production rate change is another limitation of the single loop control scheme.
A very common problem in TMP installations is the loss of pulp quality at high production rate, Murton K. D. et al., “Production rate effect on TMP pulp quality and energy consumption. J. Pulp Paper Sci., 23(8): J411–J416, 1990”. It has been suggested that this loss of pulp strength at high production rate could be attributed to an increase in refining intensity associated with a decrease in pulp residence time. Indeed at high production rate the motor load has to increase to apply a sufficient amount of energy per tonne. At higher motor load, more steam is generated. The higher rate of steam generation results in a higher steam velocity at the same specific energy, and therefore a lower pulp residence time and a higher refining intensity. This problem can be partly offset by proper adjustment of refining consistency but there is no indication in the literature on how to achieve this compensation and how to adjust refining consistencies as a function of production rate.
Although control of discharge consistency is common practice, current methods of control do not recognize the possibility to control independently refiner inlet consistency, which is solely dependant of the in-feed and flat zone dilution, production and consistency of the incoming stock; and the discharge consistency, and this creates severe limitations in the ability to change refining intensity.